The invention relates to a measuring oscillator for detecting physical parameters, more particularly pressure, which produces a signal having a frequency dependent upon the physical parameter and which comprises an amplifier fed back positively through an all pass filter. The all-pass filter comprises a phase-determining circuit element which is constructed as a bipolar circuit arrangement having a parameter-dependent ohmic measuring resistance whose resistance variation, which can be detected by the all-pass filter, is larger upon variation of the parameter value than the resistance variation itself.
A measuring oscillator of the kind described in the opening paragraph is disclosed in U.S. patent application Ser. No. 892,604 filed Aug. 4, 1986, in which the all-pass filter is connected to a measuring resistor having a temperature-dependent ohmic resistance. Although essentially this temperature-dependent ohmic resistor may be replaced by a resistor whose resistance value varies in dependence upon the variation of other physical parameters so that the measuring oscillator produces a signal having a parameter-dependent frequency, it has been found that the sensitivity is then not sufficient to detect accurately all parameter variations.